The present invention relates, in general, to wet flue gas desulfurization (WFGD) systems and, in particular, to in-situ forced oxidation WFGD scrubber application in oxygen-fired fossil fuel combustion (oxy-fuel combustion).
Air quality laws, both at the federal and state level have set increasingly stringent emission standards. Often of particular concern are sulfur dioxide and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Acidic gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes.
New technologies are addressing this problem so that fossil fuels and particularly coal can be utilized for future generations without polluting the atmosphere and contributing to global warming. The method by which acidic gases are removed with a gas-liquid contactor or other type of flue gas scrubber is known as a WFGD system. One of the technologies being developed has potential for retrofit to existing pulverized coal plants, which are the backbone of power generation in many countries. This technology is oxy-fuel combustion which is the process of firing a fossil-fueled boiler with an oxygen-enriched gas mix instead of air. Almost all the nitrogen is removed from the input air, yielding a stream that is approximately 95% oxygen. Firing with pure oxygen would result in too high a flame temperature, so the mixture is diluted by mixing with recycled flue gas. The recycled flue gas can also be used to carry fuel into the boiler and ensure adequate convective heat transfer to all boiler areas. Oxy-fuel combustion produces approximately 75% less flue gas than air fueled combustion. Due to pipeline line and use constraints, it is highly desirable to produce a flue gas that is as high in concentration of carbon dioxide and as low in concentration of nitrogen, sulfur, oxygen and water, as practical. Therefore, air introduction into the flue gas must be minimized or eliminated.
In an oxy-fuel combustion plant, a WFGD system or wet scrubber can be utilized to remove as high as 99+% of the sulfur. In this process the sulfur dioxide containing flue gas is scrubbed with the calcium or sodium based alkaline slurry or slurry reagent which may include any number of additives to enhance removal, control chemistry, and reduce chemical scale. The slurry reagent, when contacted by sulfur dioxide, results in the absorption of the sulfur dioxide by the slurry and forms sulfites which are collected in a reaction tank. Thereafter, the slurry is oxidized to cause the alkali to react with the absorbed sulfur dioxide to yield a benign and often useful product. For example, in the case of desulfurization where calcium-based alkaline slurry, such as lime or limestone, is used to absorb sulfur dioxide, air is injected into the slurry collected in the reaction tank to oxidize the aqueous sulfite into sulfate; the latter will then react with calcium ions in the slurry to form gypsum, a marketable product. It should be noted that the above reaction is exemplary, and that the teachings of this invention are not limited to the use of calcium-based slurries in a desulfurization reaction.
The technology for wet scrubbing provides gas-liquid contact in a number of differently configured systems. In recent years, WFGD systems of the type commonly used with conventional air-fired fossil fuel plants and referred to as in-situ forced oxidation type have been the preferred systems for achieving oxidation. These systems comprise two major components: the absorber or gas scrubbing zone in which the actual flue gas scrubbing takes place, and the tank or reaction zone located subjacent to the gas scrubbing zone to allow for efficient utilization of the reagent. Some oxidation of sulfite to sulfate inevitably occurs in the gas scrubbing zone and is referred to as natural oxidation, so as to distinguish it from forced oxidation in which air is sparged through the slurry in the reaction tank. The sulfites must be oxidized to sulfates in order to maintain the reaction tank generally free of scale. In a conventional forced oxidation system, the air used to oxidize the sulfite bubbles through the slurry and is released into the incoming flue gas and exits the wet scrubber for discharge through a stack.
An example of a prior art in-situ forced oxidation WFGD tower is shown at 10 in FIG. 1, wherein untreated flue gas 11 is introduced through a flue gas inlet 12 located at the lower end of a gas scrubbing zone 14 and is caused to flow upwardly and to pass through a perforated tray 16 which promotes gas-liquid contact with the absorbent liquid slurry that is draining into a reaction zone or tank 18. The now partially treated flue gas continues in its upward flow and passes through a spray area 20 where it comes into gas-liquid contact with additional absorbent liquid slurry which is being injected into the gas scrubbing zone 14 by nozzles 22, and the liquid slurry absorbs sulfur dioxide still present in the partially treated flue gas. Thence, the flue gas 11 passes through the demisters or moisture separators 23 and is discharged as treated flue gas from a gas outlet 24 located at the upper end of the gas scrubber 10. The absorbent slurry injected by the nozzles 22 flows downward through the gas scrubbing zone 14 and through the perforated tray 16 where the sulfur dioxide is absorbed into the froth created by the interaction of the flue gas and slurry on the perforated tray 16. Thence the liquid slurry, which has now absorbed the sulfur dioxide from the untreated flue gas, drains into the reaction zone or tank 18 located at the bottom of the gas scrubber 10. The in-situ forced oxidation takes place in the reaction tank 18 when air 26 is injected into the calcium-based alkaline liquid slurry 27 by a sparger 28 and/or lance (not shown) to oxidize the calcium sulfite to calcium sulfate. Also within the reaction tank 18 are one or more mixers 30 located at a minimum elevation from the bottom of the sparger 28 set by the physical dimensions of the mixer blades and the horsepower of the mixer motor. The mixers 30 agitate the slurry in the reaction tank 18 to promote oxidation through mixing in the area under the sparger 28. The oxidation air 26 bubbles through the slurry and into the gas scrubbing zone 14 where it mixes with the flue gas passing through the gas scrubbing zone 14.
About 75% of the flue gas exiting the wet scrubber of an oxy-fuel combustion plant is returned to the boiler where oxygen is introduced to produce the combustion oxidant gas, while the remainder of the flue gas is sent to a compression and cleaning system where it is prepared for transport to the point of use or sequestration. Thus, it is imperative that the carbon dioxide concentration be as high as possible with as low a concentration of sulfur, nitrogen, oxygen, and water as can be practically and economically achieved. However, at this time, there are no known economically feasible methods or systems for providing in-situ forced oxidation flue gas scrubbing without allowing the oxidation air to enter the gas scrubbing zone, as required in the oxy-fuel combustion process in order to eliminate the introduction of nitrogen in the flue gas stream.